This invention relates generally to the field of optical telecommunications, and in particular, to a method and apparatus for generating optical chirped return-to-zero (CRZ) data signals.
Optical CRZ signals, i.e., signals in which (a) the optical intensity returns to zero during each bit period, and (b) the optical phase varies (by a desired amount) within each bit period, are important for ultra long distance optical fiber communications. Such signals can be advantageously used e.g., in undersea lightwave systems. For this class of systems, CRZ has been shown to yield high tolerance towards impairments caused by fiber nonlinearities. See, for example, N. S. Bergano, xe2x80x9cUndersea Communication Systemsxe2x80x9d, in Optical Fiber Telecommunications IV B, Academic Press, 2002.
One known arrangement for generating CRZ signals uses three optical modulators, as shown in FIG. 1. The first optical data modulator 102 imprints data modulation (among many other possible formats, e.g., non-return-to-zero on-off keying (NRZ-OOK), or NRZ-differential phase shift keying (NRZ-DPSK)) on the light emitted by a continuously operating laser 101. The second modulator 103 is a return-to-zero (RZ) pulse carver that carves RZ pulses out of the optical (usually NRZ) data signal. As indicated in FIG. 1, modulator 103 is frequently realized as a dual-drive Mach-Zehnder modulator using sinusoidal drive signals at either the data rate or at half the data rate. The third modulator 104 is a sinusoidally driven phase modulator that generates the desired amount of bit-synchronuous phase modulation, i.e. a phase term
exp[jmcos(2 xcfx80Rt)],xe2x80x83xe2x80x83(1)
where m is called the modulation index.
Another known way of generating CRZ can be found in a paper by F. Liu et al., xe2x80x9cChirped return-to-zero source used in 8xc3x9710 Gbit/s transmission over 2000 km of standard singlemode fiberxe2x80x9d, Electron. Lett. 36 (16), 1399-1400, 2000, where a single Mach-Zehnder modulator is used to generate the data modulation, carve RZ pulses, and generate chirp. However, the maximum phase excursion obtainable from that scheme is limited to xcx9c0.5 xcfx80 without significantly degrading the data modulation.
In accordance with the present invention, optical chirped return-to-zero (CRZ) data signals are advantageously generated without the need for a separate phase modulator 104, as required in the prior art, by using a dual-drive Mach-Zehnder modulator (such as modulator 103 in FIG. 1) for RZ pulse carving that is driven with two typically sinusoidal signals of either unequal amplitude or unequal relative phase (i.e. with some non-vanishing phase difference).